This invention relates generally to the screen printing of inks onto substrates, and in particular, to a process of screen printing which diminishes or eliminates the Moire effect and other visual discrepancies between the original image and printed reproductions.
The screen printing process is a process of forcing ink through unblocked areas of a metal, synthetic or silk fiber screen by spreading the ink onto the screen and passing a squeegee over the screen. This process is typically used in the production of posters, signs, decalcomania, etc. Most screens are comprised of polymeric or metal threads.
It is known to screen print images on substrates through the successive application of single tones of ink. This printing process achieves tone variation by separating each process color, typically cyan, magenta, yellow and black, into fine dots of differing size on a halftone screen grid. Typically, the dots are closely spaced and arranged in parallel lines. Image coloring can be varied by superimposing the dots of the process colors. In an ideal embodiment of the printed reproduction, human vision integrates the dots into an accurate impression of the original image.
However, the use of four separate colors printed successively through a screen often results in the presence of Moire effects. A Moire effect is a repetitive interference pattern caused by overlapping symmetrical grids of dots or lines having a differing pitch or angle. Such effects are often seen as waves, shimmering or rosettes in the reproduced image.
It is understood that Moire effects are introduced by the application of the dot pattern to the screen which causes the dot shapes to be resized by the screen structure, depending on the size of the dot and the screen cell. The interference between the screen structure pattern and the dot pattern will be repeated for each color and results in Moire effects.
In order to avoid Moire effects, it is known to vary the dot arrangements for each color. For instance, U.S. Pat. No. 5,778,091 to Shibazaki et al. discloses the printing of cyan with a dot pattern axis of 15 degrees, magenta with a dot pattern axis of 45 degrees, yellow with a dot pattern axis of 0 degrees and black with a dot pattern axis of 75 degrees. While these screen angles do diminish Moire effects, discrepancies between the original image and the reproduced image still exist due to Moire.
At least some of these discrepancies are caused by positional shifts or deviations caused by the mechanical factors of the screen printer and the ink setter. Specifically, heat storage effects of these devices cause deformation in the ink receiving substrate. For each ink printing onto the substrate and setting or drying in position, the substrate is heated and cooled several times. Typically, the ink is passed under several heating/drying elements which can cause the substrate to deform or shrink by an xe2x85x9inch each pass. After an ink is set, the substrate must be realigned and re-registered with another screen for the printing of other process colors. The deformation of the substrate can significantly impair this realignment and cause dramatic positional shifts or deviations.
An improved screen printing process which addresses these problems of known screen printing processes would be an important advance in the art.
It is an object of the invention to provide a screen printing method overcoming some of the problems and shortcomings of prior art screen printing processes.
Another object of the invention is to provide a screen printing method which diminishes or eliminates interferences between dot and screen patterns.
Another object of the invention is to provide a screen printing method which reduces Moire effects to a minimum.
Another object of the invention is to provide a screen printing method which utilizes the same standard screens for each ink application.
Another object of the invention is to avoid Moire effects in screen printing without utilizing expensive, specially made screens.
Still another object of the invention is to provide a screen printing method which reduces ink-receiving substrate deformation.
Still another object of the invention is to provide a screen printing method which reduces the need for repeated registration of the substrate during the printing process.
Yet another object of the invention is to provide a precise screen printed image which results from the novel process herein described.
How these and other objects are accomplished will become apparent from the following descriptions and the drawings.
The new screen printing process is intended to result in an extremely precise and accurate screen printed reproduction of an original image. The process utilizes novel screen angles and ink drying processes which are able to reduce dot and screen pattern interference, position shifts and substrate deformation. The method of screen printing the image onto a substrate comprises the steps of (1) separating the original image into cyan, magenta, yellow and black components at a predetermined line count, (2) converting each color component into linear dot patterns on respective process films with axes at respective angles relative to the vertical, (3) transferring the linear dot patterns from the respective process films to respective screens, (4) securing the substrate in a fixed position relative to an automated substrate-guide or gripper which is able to automatically transport the substrate between ink-receiving and ink-drying positions, (5) transferring the substrate to an ink-receiving position while retaining the fixed position relative to the substrate-guide, (6) aligning a screen with the substrate, (7) applying ink to the screen so that the ink flows through the dot pattern on the screen onto the substrate, (8) transferring the substrate to an ink-drying position while retaining the fixed position relative to the substrate-guide, (9) setting the ink on the substrate through the application of light, and (10) repeating steps 5-9 for each desired process color.
The preferred substrate is a smooth material such as vinyl, other polymeric materials typically used for posters and the like, or paper coated with such material.
The precision of this screen printing process demands that the substrate be extremely smooth and able to retain ink so that the image can be properly reproduced.
It is preferred that the linear dot patterns are positioned on the process films at specific angles which have been determined to be proficient at minimizing Moire effects. One set of these angles is 82.5 degrees for cyan, 142.5 degrees for magenta, 7.5 degrees for yellow and 22.5 degrees for black. It is to be understood that all angle measurements disclosed and claimed in this patent are relative from the vertical axis, such that a xe2x80x9c3 o""clockxe2x80x9d angle would be 90 degrees and an xe2x80x9c11 o""clockxe2x80x9d angle would be 330 degrees. It is further understood that the effectiveness of these angles is due to their relationship with the geometry of the screen. Orthogonal screens have threads which run vertically and horizontally, creating rectangular screen cells. The angle measurements are effective when they are measured relative to the vertical screen threads. It is contemplated that any angular rotation of the screen threads would result in xe2x80x9cdifferentxe2x80x9d angle measurements which would minimize Moire. In accordance, the angles claimed in relation to the xe2x80x9cverticalxe2x80x9d refer to the vertical axes of the screen threads.
Another preferred set of angles determined to be extremely effective at minimizing Moire effects is 75 degrees for cyan, 135 degrees for magenta, 0 degrees for yellow and 15 degrees for black.
Each set of preferred angles utilizes the same angular displacement between pairs of dot patterns. Specifically, the angular displacement between the cyan dot pattern axes and the magenta dot pattern axes is 60 degrees. The angular displacement between the cyan dot pattern axes and the yellow dot pattern axes is 285 degrees. The angular displacement between the cyan dot pattern axes and the black dot pattern axes is 300 degrees. The angular displacement between the magenta dot pattern axes and the yellow dot pattern axes is 225 degrees. The angular displacement between the magenta dot pattern axes and the black dot pattern axes is 240 degrees. The angular displacement between the yellow dot pattern axes and the black dot pattern axes is 15 degrees.
The dot pattern of each color is created in order to properly represent the final reproduced image. A preferred dot pattern comprises either 24, 36, 45, 60, 62, 70 or 82 dot-positions per inch. A dot-position is a rectangular area in which a single dot is placed.
If the image color at a specific position does not require the presence of a certain process color, then the dot-position for that process color at that position will be empty. Conversely, if the image color at a specific position requires the full density of a process color then the dot-position for that process color will be completely filled by ink and the dot will be a rectangle. As the size of the dot increases from an empty dot-position to filled dot-position it transforms from a small substantially circular dot to a diamond shape, or ellipse. When the dot covers approximately 30-41% of the dot-position it merges with the proximate dot in the linear pattern. When the dot covers approximately 65-70% of the dot-position it merges with the proximate dot perpendicular to the linear pattern.
The shape of the dots utilized in the invention are preferably elliptical or rugby-shaped when medium sized. Rugby-shaped refers to the shape of a rugby football, which is similar to an elliptical shape though flatter at each end. Medium sized refers to the range of dot size when the dot covers approximately 20-50% of the dot-position. As the dot size moves from 20% to 50% the dot merges or joins its neighboring dot in the linear pattern.
The preferred screens for use in the claimed invention are flat screens as are known in the art. The screens are preferably orthogonally arranged such that the screen threads pass vertically and horizontally across the screen, intersecting at right angles. The resulting screen cells are rectangular, and typically square. The screen typically includes a rectangular frame which keeps the threads under tension.
During the transfer of the dot pattern to the screen from the process film, the screen cells correlating to the dot pattern on the film are unblocked and the other screen cells are blocked so that an area corresponding to the dot pattern is created through which ink is able to pass.
It is preferred that the dot pattern is transferred from the process film to the screen using photo emulsion. Typically during photo emulsion the screen is coated with emulsion that is dried. The film is aligned next to the screen so that the dot pattern angle on the film is properly oriented with respect to the vertical threads of the screen. Light is passed through the film onto the emulsion covered screen for an extended period of time. During this period the emulsion which light touches hardens and blocks screen cells. The emulsion which is shaded by the film""s dot pattern does not receive light and stays unhardened and soluble in water. The film is removed from the screen and water is sprayed over the screen, removing the emulsion which was shaded by the film""s dot pattern. In this manner, unblocked screen cells and partially unblocked screen cells substantially duplicate the dot pattern of the process film. The dot pattern can be better duplicated by screens in which the threads are thin and spaced closely together. Screens with synthetic threads are typically able to provide superior duplication since synthetic threads can be made much thinner than silk or metal threads. The screens utilized in the invention typically have 380 to 420 threads per inch.
After the film""s dot pattern is transferred to the screen, the screen is placed next to the ink receiving substrate which is fixed in position relative to a substrate-guide, preferably a gripper. It is preferred that the screen is aligned with the substrate using register marks. This is particularly important for the screens which are utilized after the first color printing. For these screens, the register mark from the first printing is aligned so that the subsequent inks are printed in the correct relationship to the first ink. The substrate need not be registered in the ink-receiving position since it is secured to the substrate-guide or gripper which automatically aligns itself precisely with respect to the ink-receiving position.
After ink is printed onto the substrate, the ink must be set or dried. The substrate-guide or gripper transfers the substrate from the ink-receiving position adjacent to the screen to an ink-drying position. The drying process is preferably performed using UV light. The application of UV light results in much less heat being absorbed by the substrate than in most prior art ink setting processes. In addition, the application of UV light can be accompanied by heat transfer away from the substrate by water-cooling the table on which the substrate lies. Furthermore, the drying process using UV light can be performed more quickly than prior art processes.
Once the most recently printed ink is set, the substrate guide or gripper transfers the substrate to an ink-receiving position where the next screen is positioned adjacent to the substrate for printing the next ink. This process is repeated for each ink which is necessary to create a satisfactory reproduced image. It is preferred that cyan is printed first, followed by magenta, yellow and black. However, this order is not compulsory in achieving satisfactory results. Often the process requires cyan, magenta, yellow and black. However, it is understood that some images may require only one, two or three of these process colors.
Because the substrate-guide or gripper is able to transfer the substrate between each ink-receiving position each ink-drying position the substrate does not need to be re-registered each time it is transferred between these positions. This allows significantly more precision than in the prior art process of manually transferring the substrate to and from each device and re-registering the substrate.